Organic compositions, such as mineral oils and lubricating compositions, are subject to deterioration by oxidation and in particular are subject to such deterioration at high temperatures in the presence of air. This deterioration often leads to buildup of insoluble deposits that can foul engine parts, deteriorate performance, and increase maintenance. This is particularly the case for lubricating oils used in jet aircraft where wide temperature ranges and extreme operating conditions are likely to be encountered. Proper lubricating of aircraft gas turbines, for example, requires ability to function at bulk oil temperatures from as low as −60° C. to as high as 230°-280° C. Such an extreme temperature range places unique demands on the characteristics of the lubricant. Aviation jet turbine lubricants require superior thermal and oxidative stability, good viscosity-temperature characteristics (high VI), low volatility and a low pour point. Organic hydrocarbon-based oils are typically not robust enough to satisfy these requirements. Thus, aviation applications have relied on the superior performance characteristics of synthetic ester lubricants.
Ester base lubricating oil compositions prepared from polyols such as neopentyl glycol, trimethylolpropane or pentaerythritol, and a mixture of fatty acids and containing selected additive combinations are well known. These lubricants are functional over a wide temperature range and exhibit good thermal and oxidative stability. An ester base lubricant composition that will operate under more severe conditions, however, is a major goal of lubricant manufacturers. This invention addresses that continuing need by synthesizing dispersants that are compatible with synthetic ester base stock and which will improve the deposit control performance of the lubricant.
Conventional dispersants are oil-soluble additives that improve the deposit control performance of a lubricant by suspending deposit-forming precursors in the oil. These additives typically have a molecular structure consisting of a large non-polar hydrocarbon portion connected to a polar headgroup with the polar headgroup being comprised of nitrogen- and/or oxygen-containing functional groups. Dispersants solubilize deposit-forming precursors by incorporating the precursor into a micelle, with the polar component of the dispersant interacting with the deposit and the non-polar portion interacting with the surrounding oil. This interaction keeps the potentially harmful deposit precursors suspended.
Dispersants are critical to deposit control in gasoline and diesel crankcase lubricants and are often referred to as PIBSA-PAM type dispersants (PolylsoButenyl Succinic Anhydride-PolyAMine). Many patents involve the use of PIBSA-PAM-type dispersant chemistry.
U.S. Pat. No. 4,655,946 to Exxon Research and Engineering discloses the use of very low levels of PIBSA-PAM dispersant in a sea water resistant turbo oil.
U.S. Pat. No. 3,914,179 to BP discloses the use of an amine with up to 24 carbon atoms in synthetic ester base stocks of the polyol type.
U.S. Pat. No. 4,253,980 to Texaco discloses a dicarboxylic acid ester lactone with pyridinium function on the ester. The pyridinium function uses a salt of halide, carbonate, sulphite, borate, carboxylate or phosphate as the counter ion to produce a quaternary ammonium salt.
U.S. Pat. No. 4,239,636 to Exxon Research and Engineering discloses functionalized pentaerythritol esters incorporating polar functional groups containing sulfur.
Conventional dispersants are not practical in aviation turbine lubricant formulations because they are synthesized in a mineral oil base stock. Mineral oil base stock is inherently less stable and would contaminate the aviation turbine oil product. In addition, they demonstrate poor miscibility with ester base stocks. Thus, Applicants have designed and synthesized dispersant additives that are compatible in ester base stocks. These dispersant additives, when added to a synthetic ester base stock turbo oil enhance the cleanliness of the turbo oil.